“These prisoners were held unjustly by a regime that continues to threaten the peace and security of the Middle East,” Clinton added. “Another American, Bob Levinson, still isn’t home with his family.”

Clinton said, as president, her policy toward Iran would be to “distrust and verify.

The former secretary of State said the U.S. should not “thank” Iran for releasing prisoners unjustly held or for following through on its obligations.

“The treatment of our Navy sailors earlier this week was offensive, including the release of demeaning and provocative videos,” she added, referring to the brief capture of 10 sailors who inadvertently crossed into Iranian waters.

But despite her concerns, Clinton said she was happy to hear of the release of the four Americans.

“I am greatly relieved by the safe return of American prisoners from Iran,” Clinton said in a statement. “Their families and our country have waited and prayed for this day to come.”

Clinton also said the nuclear agreement marks an “important achievement of diplomacy” between Iran and the international community.

“Implementation marks an important step forward in preventing Iran from obtaining a nuclear weapon,” she said. “Iran has dismantled centrifuges, disabled a reactor, and shipped out almost all of its enriched uranium.

“These are important steps that make the United States, our allies, and the entire world safer,” she added. “I congratulate President Obama and his team, and I’m proud of the role I played to get this process started.”